Shock waves are traveling pressure fluctuations that cause local compression of the material through which they travel. When traveling through a gas, such as air, shock waves produce increases in pressure, referred to as “overpressure”, along with increases in temperature. They also accelerate gas molecules and entrained particulates in the direction of shock wave travel. Shock waves produced by explosions also release substantial amounts of thermal and radiant energy.
Shock waves can cause significant damage to both humans and mechanical structures. The overpressure caused by a shock wave is one source of such damage. As indicated in FIG. 1A, panels of sheet metal buckle due to an overpressure as low as about 1.1-1.8 psi. Concrete walls fail at overpressures between about 1.8-2.9 psi, and most buildings are completely destroyed by over pressures of about 10-12 psi. As indicated in FIG. 1B, an overpressure of greater than about 50 psi creates sufficient body disruption to severely injure, and in many instances, kill a human being.
Traditionally, various chemical and mechanical approaches have been employed to attenuate, deflect and/or diffract shock waves to mitigate the damage they cause. Prior art approaches include, for example, solid barriers, mechanical venting, chemical agents, aqueous foams, solid foams, solid beads, and combinations thereof. All of the prior art approaches for shock wave mitigation suffer from significant drawbacks, such as being toxic to humans, too heavy, too bulky, not easily transportable, and not usable in a wide variety of applications.
For example, one prior art approach employs solid barriers for deflecting incident and/or attenuating shock waves, and for providing protection from fragments and thermal effects. Such solid barriers suffer from several shortcomings. Where protection of large areas from powerful shock effects is necessary, structures must be massive and are thus inherently immobile, expensive and time consuming to erect.
Another prior art approach employs blast mats. A disadvantage of blast mats is that they are heavy and bulky. When not being used, they require large amounts of storage, and due to their weight and bulk are not easily moved from storage to a location where they are needed. Also, blast mats provide little acoustic damping.
Mechanical venting is widely employed for mitigating blast overpressure in containment structures (e.g., grain silos, explosive material handling rooms, and the like). The vents normally constitute part of a containment wall. Besides reliability and response time problems, venting requires facilities to be designed such that overpressure release will not endanger personnel or nearby structures. Venting does not provide protection from a blast originating in an open, uncontained environment. Venting also cannot be employed where hazardous materials may be released, and does not provide significant shock wave attenuation.
Chemical agents suppress shock waves by extinguishing or interrupting the combustion process that generates them. Such agents include, for example, carbon dioxide and halogenated carbon compounds (“halons”), which may be gaseous or liquid at the time of application, and dry powders, most of which are salts of ammonium or alkali metals, such as sodium and potassium. Chemical combustion-extinguishing agents are generally effective in confined spaces, with powders also being effective in unconfined environments. However, chemical agents currently available for fire and explosion suppression typically have toxic effects upon humans at the concentrations required to be effective. Also, aside from removing the source of the shock wave, they do not provide any significant attenuation for the shock wave caused by the initial explosion.
Aqueous foams have been proven to be capable of providing significant shock wave attenuation. Aqueous foams rely, in part, on scattering and dispersing the pressure waves at the bubble/cell walls. Also, the displacement of the bubbles in the aqueous foam absorbs substantial energy. Additionally, shock waves propagating through aqueous foams create turbulent flow fields, which also dissipates substantial amounts of energy, particularly when reflected waves travel through the turbulent medium. Typically, aqueous foam for pressure wave attenuation is deployed either in an unconfined deluge or as a filler material in solid confining walls. High-capacity foam deluge systems have been used for perimeter security and for flooding buildings to provide explosion protection from bombs. Aqueous foam-filled containers have also been used for safe removal and disposal of explosives. Variants of the foam-filled container concept have been developed as noise-attenuation devices (“silencers”) for the muzzles of firearms and large naval guns. One drawback of aqueous foam is that it requires a foam generation system and/or a large bulky supply of foam to be stored wherever it is to be deployed. Solid foams have also been employed for shock wave attenuation. However, solid foams have proven not to be as effective as aqueous foams at attenuating shock waves. Turbulent flow fields are not generated within solid foams, and bubble displacements cannot occur.
According to another prior art approach, loosely packed beads are employed to attenuate shock waves. The beads, unlike the solid foam bubbles, are capable of relative displacement in the nature of a fluid. In such a form, the beads act similarly to the bubbles in an aqueous foam. Specifically, transmitting shock waves are scattered and dispersed at the bead surfaces, and the displacement of the bead mass absorbs substantial energy. In some implementations, the beads are made to resist displacement to a limited extent (below the degree where the bead mass would act more as a rigid panel than a fluid) to further attenuate the shock wave. However, the solid bead approach suffers from the drawback that it is typically employed with a solid rigid frame for containing the beads, foam or a combination thereof.
Because prior art approaches to shock wave attenuation suffer from significant deficiencies, including being too heavy, not being easily transportable, taking up too much storage, they are not practical for many applications where explosion hazards are present, such as, battle field conditions where structures need to be easily erected, dismantled and transported. The deficiencies also render them impractical for personal body protection for soldiers, and for motor vehicle protection.